- Reginald Owen - Dr. Becquerel
- Van Johnson - Reporter
- Margaret O'Brien - Irene Curie - Age 5
- Mariska Aldrich - Tall Woman (uncredited)
- Ruth Cherrington - Swedish Queen (uncredited)
- Ray Collins - Lecturer (voice) (uncredited)
- Guy D'Ennery - Professor (uncredited)
- George Davis - Cart Driver (uncredited)
- Harold De Becker - Professor (uncredited)
- Tay Dunn - Man at Accident (uncredited)
- William Edmunds - Cart Driver (uncredited)
- Al Ferguson - Man at Accident (uncredited)
- Edward Fielding - Board Member (uncredited)
- Howard Freeman - Prof. Constant (voice) (uncredited)
- Lisa Golm - Lucille (uncredited)
- Ilka Grüning - Seamstress (uncredited)
- Lumsden Hare - Professor Roget (uncredited)
- Teddy Infuhr - Son (uncredited)
- James Kirkwood - Board Member (uncredited)
- Isabel La Mal - Woman at Accident (uncredited)
- Gene Lockhart - Bit Part (uncredited)
- Miles Mander - Businessman (uncredited)
- George Meader - Singing Professor (uncredited)
- Alan Napier - Dr. Bladh (uncredited)
- Moroni Olsen - President of Businessman's Board (uncredited)
- María Pagés - Woman at Accident (uncredited)
- Gigi Perreau - Eve at 18 Months (uncredited)
- Francis Pierlot - Monsieur Michaud (uncredited)
- Nita Pike - Woman at Accident (uncredited)
- Dorothy Raye - Nurse (uncredited)
- Almira Sessions - Madame Michaud (uncredited)
- Arthur Shields - Businessman (uncredited)
- Wyndham Standing - King Oscar (uncredited)
- Ray Teal - Driver (uncredited)
- Charles Trowbridge - Board Member (uncredited)
- Michael Visaroff - Proud Papa (uncredited)
- Marek Windheim - Jewelry Salesman (uncredited)
- Frederick Worlock - Businessman (uncredited)
- Eustace Wyatt - Doctor (uncredited)
- Mervyn LeRoy
- Paul Osborn
- Hans Rameau
- Sidney Franklin
- E.J. Mannix
- Herbert Stothart
Greer Garson hace el papel de Marie Curie.
Walter Pidgeon hace el papel de Pierre Curie.
Henry Travers hace el papel de Eugene Curie.
Albert Bassermann hace el papel de Professor Jean Perot.
Robert Walker hace el papel de David Le Gros.
C. Aubrey Smith hace el papel de Lord Kelvin.
May Whitty hace el papel de Madame Eugene Curie.
Victor Francen interpreta a President of University en la película.
Elsa Bassermann interpreta a Madame Perot en la película.
Reginald Owen interpreta a Dr. Becquerel en la película.
Van Johnson hace el papel de Reporter.
Margaret O'Brien interpreta a Irene Curie - Age 5 en la película.
James Hilton interpreta a Narration Spoken By (voice) en la película.
Mariska Aldrich interpreta a Tall Woman (uncredited) en la película.
Tony Carson interpreta a Man at Accident (uncredited) en la película.
Ruth Cherrington interpreta a Swedish Queen (uncredited) en la película.
Ray Collins interpreta a Lecturer (voice) (uncredited) en la película.
Guy D'Ennery interpreta a Professor (uncredited) en la película.
George Davis interpreta a Cart Driver (uncredited) en la película.
Harold De Becker hace el papel de Professor (uncredited).
Franz Dorfler interpreta a Assistant Tailor (uncredited) en la película.
Justine Duney interpreta a Woman at Accident (uncredited) en la película.
Tay Dunn hace el papel de Man at Accident (uncredited).
William Edmunds hace el papel de Cart Driver (uncredited).
Nestor Eristoff interpreta a Board Member (uncredited) en la película.
Al Ferguson interpreta a Man at Accident (uncredited) en la película.
Edward Fielding interpreta a Board Member (uncredited) en la película.
Howard Freeman interpreta a Prof. Constant (voice) (uncredited) en la película.
Linda Lee Gates hace el papel de Perot Granddaughter (uncredited).
Marie Louise Gates interpreta a Perot Granddaughter (uncredited) en la película.
Lisa Golm interpreta a Lucille (uncredited) en la película.
Ilka Grüning interpreta a Seamstress (uncredited) en la película.
Lumsden Hare hace el papel de Professor Roget (uncredited).
Teddy Infuhr interpreta a Son (uncredited) en la película.
James Kirkwood interpreta a Board Member (uncredited) en la película.
Isabel La Mal hace el papel de Woman at Accident (uncredited).
Gene Lockhart interpreta a Bit Part (uncredited) en la película.
Miles Mander interpreta a Businessman (uncredited) en la película.
George Meader hace el papel de Singing Professor (uncredited).
Dickie Meyers hace el papel de Master Michaud (uncredited).
Noel Mills interpreta a Wedding Guest (uncredited) en la película.
Leo Mostovoy interpreta a Photographer (uncredited) en la película.
Alan Napier interpreta a Dr. Bladh (uncredited) en la película.
Moroni Olsen interpreta a President of Businessman's Board (uncredited) en la película.
María Pagés hace el papel de Woman at Accident (uncredited).
Gigi Perreau interpreta a Eve at 18 Months (uncredited) en la película.
Francis Pierlot interpreta a Monsieur Michaud (uncredited) en la película.
Nita Pike interpreta a Woman at Accident (uncredited) en la película.
Dorothy Raye interpreta a Nurse (uncredited) en la película.
Almira Sessions interpreta a Madame Michaud (uncredited) en la película.
Arthur Shields interpreta a Businessman (uncredited) en la película.
Wyndham Standing interpreta a King Oscar (uncredited) en la película.
Ray Teal interpreta a Driver (uncredited) en la película.
Charles Trowbridge interpreta a Board Member (uncredited) en la película.
Michael Visaroff interpreta a Proud Papa (uncredited) en la película.
Marek Windheim hace el papel de Jewelry Salesman (uncredited).
Frederick Worlock hace el papel de Businessman (uncredited).
Eustace Wyatt interpreta a Doctor (uncredited) en la película.
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